A typical photovoltaic cell has a photosensitive substrate and converts light into energy. Conductors, also known as tabs, are attached to the photovoltaic cells and are typically made of flat wire or ribbon wire. Flat wire is pulled from a spool, straightened, flattened and pretensioned then cut to length to form individual tabs. In some cases, a Z-bend may be formed in the ribbon wire to accommodate a transition from the front of one cell to the back of an adjacent cell. The conventional practice results in a trade-off between the surface area covered on the front side of the photovoltaic cell and the net series resistance of the photovoltaic cell string determined by the cross section of the tab. As the ribbon width is increased and, to a lesser extent, as the ribbon thickness is increased, the amount of surface area of the photovoltaic cell that is occluded increases, which tends to sacrifice optical efficiency for reduced series resistance. For example, a 156 millimeter (mm) cell with 2 mm tabs may have approximately 2.56% of the light blocked but may have significant series resistance. Increasing the tab to 3 mm may reduce series resistance by approximately 33 percent but increases the area blocked by 50 percent.
The use of a conventional fire-through metalization process can be used to form front metal contacts on a photovoltaic cell. A wider area of metalization forming a contact on the front of the cell in order to accommodate wider tabs may promote localized heating during the process, which may increase the degree to which shunts, that is, places where the semiconductor junction is damaged, are formed. On the other hand, reducing tab resistance by increasing thickness has the effect of increasing the overall thickness of the string of cells which can make the subsequent lamination process more difficult and induce additional mechanical stress on the cells. Commonly, a solder contact is produced by scavenging solder previously applied to the ribbon wire. This solder is typically applied to both sides of the wire resulting in increased thickness while more than half of this material is not used since it is on a surface that does not come into contact with a cell. Since lead free solders contain a substantial amount of silver, this is an objectionable expense. Generally, in order to accommodate variation in the width of ribbon wire as supplied, the front contacts of a cell are made at least as wide as or somewhat wider than the ribbon wire, with a minor loss of cell efficiency. An additional problem is that tabs experience thermal expansion and contraction during any thermal method of attaching tabs to cells which can result in stresses applied to the cell due to differential thermal expansion and contraction. This is generally ameliorated by pre-tensioning the wire during the tabbing-stringing process but this is generally considered a weak solution which is less effective as the thickness of solar cells is reduced.
Typically, tabs are formed from flat wire; however, another option may be to form the wire with a faceted top surface such that incident light is reflected at a sufficient angle to be trapped inside the module, by total internal reflection, and reflected onto the active area of the cells (for example Light-Capturing Ribbon supplied by Ulbrich). However, this option may have a negative impact on the attachment of tabs to back contacts where the facets can serve no optical purpose but provide a more difficult surface to attach to the back contacts of the cell. Furthermore, this approach results in a thickening of the tab along its entire length in order to form facets while maintaining the desired conductor cross section where only the front contact portion serves a purpose.
It is, therefore, desirable to provide systems and methods intended to overcome at least some of the problems of conventional systems and methods and improve the forming of the tabs for photovoltaic cells and the tabs themselves.